Geothermal energy is defined as “heat from the earth”. The earth's heat is a “renewable resource” as defined by The National Energy Policy Act of 1992 and the Pacific Northwest Electric Power Planning and Conservation Act of 1980 and is estimated by the U.S. Geothermal Industry for the Renewable Energy Task Force (1997) to be “equivalent to 42 million megawatts of power”.
The potential for using heat energy found within the earth, sometimes referred to as “heat mining”; either for direct use or for generation of electricity has been a subject of significant interest over the years. If heat mining could be efficiently and safely employed, geothermal resources could represent a nearly inexhaustible source of non-fossil, non-nuclear fuel.
It should be noted that other so called “green” or “clean” alternative technologies tend to have practical limitations. Electrically powered vehicles currently require electricity for recharging to be generated by fossil fuel or nuclear fuel powered generating facilities.
Pollution is therefore not eliminated and if employed on a large scale demand will drive the cost of electricity to unreasonable levels given current technological limitations.
Large scale implementation of solar collector fields or wind turbine fields will at great cost exert their own environmental impacts, which are at this point undetermined. Additionally, there is the unreliability factor in that the wind doesn't always blow and the sun doesn't always shine and certainly doesn't shine at night. Hydrogen powered vehicles are an exciting alternative, but the large volume of fuel storage required per vehicle makes them impractical at present.
Natural gas powered vehicles are a good stop gap, but natural gas is not renewable, it's expensive and burning it does contribute to so-called green house gases. A natural gas distribution network would have to be created as it does not presently exist in a form suitable for safely fueling individual vehicles on a large scale. Ethanol fuels on any large scale are an environmental and economic disaster worse than gasoline or diesel fuel. Ethanol production is already causing food shortages, which will only worsen as production is increased. Ethanol by the time it is actually grown, harvested, processed and used contributes more green house gases to the atmosphere than gasoline or diesel fuel.
Utilization of geothermal energy in North America goes back 10,000 years. The Paleo-Indians were known to use geothermal hot springs for warmth and cleaning by direct use. More recently geothermal energy has been utilized for a number of things such as producing electricity, heating buildings, streets and sidewalks and for food processing.
With regard to generation of electricity the first sizable geothermal electricity generating plant was constructed in Larderello, Italy in 1904. The Larderello plant is still operational today. The first U.S. commercial geothermal power plant, called “The Geysers” in California began operation in the 1960's. The Geysers system is today considered to be the largest single, renewable energy source in the world.
The United States currently adds up to 2,800 megawatts of electricity to the grid annually via geothermal means. This is a small portion of the annual U.S. production. Uncertainty regarding the availability, renewability and cost of conventional fossil fuels along with potential thermal and environmental pollution, in conjunction with national security concerns, makes the production of electricity using geothermal heat an attractive alternative. This alternative has not, unfortunately, been without its own issues and costs.
Generally speaking, the temperature of the earth's crust increases with depth at an average rate of 3° C. per 100 meters of depth. To harness this heat energy at useful temperatures, say higher than 150° C. under normal crust conditions requires drilling to great depth with a significant cost of investment. To reach below ground temperatures of 300° C. to 400° C. is even more costly. There are, however, geographic regions having pronounced geologic, or more importantly, geothermal anomalies, which can provide access to usefully high temperatures at relatively shallow depths. Heat energy can be mined in these areas at significantly reduced cost if done in an appropriate and safe manner.
Employing current relevant art forms geothermal heat energy is typically extracted by pumping directly from an underground geothermal reservoir or indirectly from the geothermal source, say hot dry rock (HDR), by pumping extraneous water under high pressure through the hot rock to create a below ground reservoir. This may involve first fracturing the rock to make it more permeable should the existing rock not be in a sufficiently fractured condition. Water is typically pumped into the reservoir through a supply well. Hot water or steam is removed from the reservoir through a return well. The water is heated by the geothermal source as it passes through the fractured hot rock reservoir from the supply well to the return well. This type of system is currently employed in Iceland.
Via the return well the heated water is pumped or in the case of steam rises convectively to the surface where its useful thermal energy is used directly or can be converted to electrical energy. After using, the water may be re-circulated back to the reservoir to mine more heat or may be wasted. These fluids can be used for direct heating of structures, for food processing or more frequently for the generation of electricity using a steam turbine. Depending primarily on available temperature the type of electric generating plant may be a flash, dry steam or binary plant. The Geysers in California is a dry steam field, which is quite rare. In a dry steam field, steam, not fluid shoots up the well and powers the turbine.
What are some of the issues? A number of patents are referenced at the beginning of this specification. These patents clearly demonstrate the types of difficulties encountered in attempting to mine geothermal heat energy. The references cited are informational only and do not infer any dependent relationship to claims made herein. As mentioned above geologic formations yielding high temperature rock and/or fluids at shallow depths are economically attractive candidates for heat mining. Unfortunately, many, if not most of these formations are found in tectonically active locations, some of which may experience significant faulting along with earthquake activity and in the case of active volcanoes, may experience flows of molten igneous material or airborne ash and debris. Applying the above mentioned technology inappropriately within such a tectonically active area would be unsafe and from an investment standpoint would have to be characterized as high risk. The careful selection of safe geothermally productive sites is critical to the future exploitation of geothermal energy in any form.
A second issue in various locations is a legal one. Some geothermally active formations are located within scenic areas or even within national parks such as Yellowstone National Park. These areas are inviolate. For example per the Geothermal Steam Act of 1970 and as amended in 1988 “certain lands, including lands within units of the National Park System are closed to federal geothermal leasing”.
Another significant and potentially costly issue involves the direct use of geothermal fluids as is typical today. These below ground geothermal fluids; sometimes called brines due to their high mineral content can contain other potential contaminants such as sulfur, boron, mercury and arsenic. Geothermal fluids can be highly corrosive and the direct use thereof can be damaging to equipment. This has resulted in a number of new patents over the years directly related to improving equipment usability and reliability as well as to develop processes for removing the contaminants prior to use or after using. The direct use of fluids pumped from geothermal reservoirs has been costly from an anti-corrosion standpoint, but perhaps even more so by limitations as to actual extent of “renewability”, which in some ways becomes the most significant issue regarding future investment in geothermal energy.
There is no question that underground geothermal reservoirs are completely renewable as long as rain continues to fall above ground and the earth's core remains hot. There are, however, serious limitations on the rate at which these insitu fluids can be removed from the reservoir and replenished. If geothermal fluid is removed from the reservoir at a rate higher than that at which the aquifer can recharge itself; the fluid level within the reservoir will drop over time, resulting in a necessary increase in pumping cost at the least and in the worse case a diminishment of the resource for useful purposes.
The situation described above has occurred at The Geysers in California over the last two decades resulting in less fluid being pumped and less electricity being produced. The Geyser problem was largely resolved by the world's first wastewater-to-electricity system (Southeast Geysers Effluent Pipeline), which conveys water from Clear Lake and wastewater effluent from Lake County, both of which are used to replenish the Geysers underground geothermal reservoir(s) so as to increase electric producing capacity. The Geyser's situation then was dealt with by bringing in an outside water source to replenish the receding reservoir levels. This has been an expensive solution.
Another relevant art form simply fractures the hot dry rock if not already fractured sufficiently and pumps water down into the rock fractures under high pressure to be heated and then extracted. This process was touched on earlier. This has been a successful method, but has raised serious questions regarding potential long term environmental effects of artificially injecting large quantities of water into what has historically been dry rock strata. Such systems are employed in Iceland and have been in use for a long period of time both in the generation of electricity and for direct use in heating and food processing.
The list of problems historically associated with the use and harvesting of geothermal energy sources is not particularly long. It is none-the-less a serious list posing rather costly solutions.
In determining whether a geothermal electric generating plant should be built and in determining the size of the plant to be built; a cost-benefit analysis must be performed. A major component of this analysis is the estimated future revenue to be generated through the sale of electricity produced both to pay back the initial capital investment as well as to provide a reasonable dividend to shareholders. Uncertainty with regard to future reservoir levels means uncertainty with regard to future electricity production, which means uncertainty with regard to future revenue, which in turn means increased risk of investment. The investment required is substantial.
Given the uncertainty of maintaining underground reservoir levels and the high cost of recharging levels with pumped surface water; it would reduce risk significantly if there were a way to by-pass or eliminate the direct use of geothermal brines altogether.
One last complication regarding direct use of geothermal fluids is environmental. Vaporization of these geothermal fluids to drive a steam turbine and subsequent condensation back to liquid tends to bring contaminants out of solution. The direct discharge of this untreated brine back into the reservoir or to a local river or lake can result in environmental pollution. Treatment of this brine then becomes an additional cost.
Direct discharge of un-cooled geothermal brines to a lake or river can result in thermal pollution. This is serious as an increase as small as 4° C. within a water course can be fatal to many fish and harmful to aquatic plants. Some geothermal electricity generating plants reinject the treated brine back into the underground reservoir. Some plants cool and waste the treated brine to a local water course. In either case both treatment and cooling of vast quantities of geothermal fluid add significantly to the cost of electricity production by existing geothermal means. Added cost is always added financial risk.
Environmental concerns regarding the discharge of so-called green house gases to the environment are continuing to increase, at least within developed nations. Jurisdictional regulation continues to impose increasingly rigorous standards intended to reduce these discharges. The electrical power industry is said to contribute approximately ⅓ of these so-called green house gases as a result of fossil fuel combustion. At present, more than fifty percent of electric power generation is fueled by fossil fuels such as coal and natural gas. It would be helpful if an effective means of providing phased improvements were available to assist power generating plants in meeting these more stringent regulatory requirements.